1. Field of the Invention
This invention relates to an electric furnace, particularly an arc furnace, with a liquid cooling device for thermally highly stressed construction parts of the furnace cover and with essentially horizontal cooling pipes which carry liquid and empty in a cooling liquid distributing conduit on a cover ring.
2. Description of the Prior Art
Such a liquid-cooled furnace cover is known from the publication Clesid; Croupe Creusot Loire; "Panneaux et Voutes Refroidis" [French-Cooled Panels and Vaults], undated. This cover consists of pipe bundles which are basically slightly arched, radially positioned parallel and adjacent to each other and extend from the central cover opening to the cover edge. The liquid is supplied through a ring line in the wall of the central cover opening and the liquid is removed through a ring line in the cover ring.
A protective layer of fireproof material which is comparatively thin compared to the thickness of a customary arc furnace cover consisting of fireproof construction material is applied to the part of the cooling pipes which faces the inside of the furnace. This protective layer both protects the cooling pipes from heat radiation and prevents too much heat from being removed from the furnace area.
It is possible to save fireproof material when such liquid-cooled furnace covers are used, but there is also the danger, given the relatively thin protective layers, that they can loosen at certain spots in an uncontrolled fashion, e.g. by mechanical action when the cover is raised or lowered, by thermal tensions inside the layers as a result of inhomogeneous heat radiation, unequal cooling action or when the furnace cover cools down. The heat transfer and thus the heat loss is particularly great at the exposed areas where the metal surface of the cooling pipes is directly irradiated by the arcs. Moreover, the non-protected areas of the cooling pipes receive a greater thermal stress then the other, protected part of the cooling pipes facing the inside of the furnace. The non-protected areas in the furnaces can become greatly heated by the smelting in two-shift or three shift operation which is normally continuous in steel plants and foundries, without being noticed by the personnel. In the most unfavorable instances, if, for example, the cooling conditions are unsatisfactory, these hot areas can lead to perforations which can entail severe consequences.
Detection systems for monitoring cooling systems are complicated and expensive. If there were an indication of trouble, the furnace cover would then have to be taken out of operation so that the defective areas could be repaired. In addition, cooling pipes which face the inside of the furnace and are covered only with a relatively thin protective layer are constantly exposed to forces of expansion and construction due to sharp variations of temperature, even though they are given a stress-free annealing before assembly. These forces exert thermal stresses on the cooling pipes which are transferred to the welding seams connecting the cooling pipes to the cover ring, and tears can form under constant stress which then result in a breakthrough of water. Moreover, the weight of a liquid-cooled cover which consists of lined-up cooling pipes is great, and special measures must be taken when it is transported and placed on the furnace vessel.